Attempts to use liquid crystal elements utilizing electrooptic effect of twisted nematic (TN) liquid crystals (referred to as "TN liquid crystal elements" hereinafter) as display elements of animation display devices have been heretofore made.
The TN liquid crystal elements, however, have a long (slow) electrooptic response time of several tens milliseconds. Therefore, the display devices using the TN liquid crystal elements hardly follow animation of quick motions, even when the display devices are driven by the use of active elements. Further, the angle of field of an image displayed by the use of the TN liquid crystal element is narrow, and therefore when the image displayed by the use of the TN liquid crystal element is observed with an angle beyond the fixed angle of field, a problem of an image with reversed gradation or a problem of change in color tone of the image takes place.
On the other hand, display systems not using the nematic liquid crystals but using smectic liquid crystals having higher order than the nematic liquid crystals have been proposed. For example, there are a display system using a surface stabilized ferroelectric liquid crystal, which has been reported by Clark and Lagerwall (Appl. Phys. Lett. 36, 899 (1980)), and a display system using a liquid crystal of chiral smectic CA phase known as one of antiferroelectric phases, which has been proposed by Chandani, et al. (Jpn. J. Appl. Phys., 27, L1279, 1988). These display systems can be driven with an electrooptic response time shorter by two or three figures than that of the display systems using the nematic liquid crystals, and besides, a wide angle of field can be obtained in these systems.
In the former system, however, only binary display is theoretically possible and gradation display is generally difficult. In the latter system, though the gradation display is feasible, the threshold value voltage is generally high, so that it is difficult to drive the system by the use of common active elements such as TFT.
Differently from the above-mentioned systems, a display system using a smectic liquid crystal having an applied voltage-light transmittance relationship shown by a V-shaped hysteresis curve, namely, a display system using "thresholdless antiferroelectric liquid crystal", has been recently reported by Inui, et al. (the 21st Liquid Crystal Forum 2C04 (1995)) and Tanaka, et al. (the same forum, 2C18), and has been paid much attention. This system utilizes such a phenomenon that the directions of optic axes of the liquid crystal are uniformly changed when a voltage is applied to a liquid crystal cell wherein the liquid crystal is filled and so orientated that the smectic layer of the liquid crystal becomes perpendicular to the substrate. In this system, the voltage necessary for the response of the liquid crystal is extremely low. This system can be driven by a common active element such as TFT, and is proposed as a system capable of displaying intermediate tone (Fukuda, Asia Display '95 S6-1). Other than this liquid crystal, there is known, as a liquid crystal composition showing such properties as mentioned above, a liquid crystal composition containing a liquid crystal compound having a ferroelectric phase and an antiferroelectric phase in its phase series (referred to as an "antiferroelectric liquid crystal" hereinafter) in an amount of not less than 40% by weight and containing a liquid crystal compound having a ferroelectric phase but having no antiferroelectric phase in its phase series (referred to as a "ferroelectric liquid crystal" hereinafter), as described in Japanese Patent Laid-Open Publication No. 59624/1997. At present, however, there are very few useful thresholdless antiferroelectric liquid crystal compositions.